A high efficiency and a high power output have been demanded for the power supply, and a switching power supply device is used to meet the demands. As the switching power supply device, a transistor is controlled to be turned on/off by a switching signal, and a duty ratio of the switching signal is changed so that the output power is constantly maintained regardless of a load. Therefore, the switching signal is referred to as a pulse width modulation (PWM) signal. In detail, a difference between an output voltage and a reference voltage is calculated and the duty ratio of the switching signal is changed depending on the difference to control the output voltage to be close to the reference voltage.
In the switching power supply device, a direction of flowing current is switched by controlling the transistor to be turned on/off by the switching signal so that significantly large switching noise is caused. The switching noise acts as an electromagnetic field radiation noise to the outside so as to cause electromagnetic interference in surrounding electronic devices. Therefore, in the switching power supply device, the noise needs to be suppressed from occurring.
In order to suppress the noise from occurring, it is widely known that a spread spectrum signal is used. The spread spectrum signal is a signal whose period varies by several percent and, for example, an IC including a dedicated spread spectrum clock generator which generates a spread spectrum clock is provided.
It is known that in the switching power supply device, the period of a switching operation is changed with the switching signal as a spread spectrum signal, thereby reducing a noise amount at a switching frequency. Therefore, it may be suggested to mount the dedicated IC including the spread spectrum clock generator on the switching power supply device to generate a spread spectrum switching signal. However, the dedicated IC including the spread spectrum clock generator is expensive so that a cost for the switching power supply device is increased.
Therefore, it is considered that a controller configured to control the switching power supply device generates the spread spectrum switching signal. However, since a period of the spread spectrum switching signal varies, when a difference between the output voltage and the reference voltage is obtained and a duty ratio of the switching signal is changed in accordance with the difference, an output voltage of the switching power supply device undesirably varies.
Even though it is considered to correct the duty ratio so as to reduce the variation of the output voltage, the period of the spread spectrum switching signal is changed at every switching period, so that an operation for correction needs to be frequently performed. In order to perform the operation, a high performance and high speed operating circuit is required and a cost for the switching power supply device is increased.
The following are reference documents.    [Document 1] Japanese Laid-Open Patent Publication No. 2013-038916 and    [Document 2] Japanese Laid-Open Patent Publication No. 2009-100607.